The Finger Phenomenon

We all know androgens and estrogens as sex hormones, right? You know, those chemicals that regulate reproductive behavior and ensure the continuation of species. There is definitely behavioral evidence of the biological importance of these steroid hormones, but could there be a way to quantitatively measure exposure to them? There is research that says yes, or at least, possibly.

Based on correlational data, the hypothesis that digit ratio can be used as a marker of prenatal hormone exposure has been floating around. Supposedly, having a ring finger (fourth digit, “4D”) that is larger than your second finger, “2D” is indicative of androgen exposure, and more masculinization. In contrast, having a second digit that is equal to in length or longer than your fourth digit is the more feminine phenotype, resulting from less androgen exposure.This, however, is a largely over-simplified perspective.

In a 2009 study, Sheri Berenbaum and colleagues compared the digit ratios of normal males (n=66) and females (n=90) with those of women who suffered from complete androgen insensitivity syndrome (CAIS, n=16). If the hypothesis was correct, these women would be expected to have high 2D:4D ratios, and they did – as compared to men – but not relative to the control women. Additionally, the study found no predictability of group membership (control male, control female, CAIS female) based on digit ratio, whether the subjects were given equal probability of being in either group or basing the probability on group size…group sizes which were oddly varied in this particular study.

Another thing I found peculiar was the method used to obtain the finger measurements – rather than directly measuring the participants’ fingers, the researchers used photocopies of their hands, which would seem to allow for more variation (pressure, straightening of fingers, etc.) Interestingly, the paper mentions that male and female skeletons do not show digit ratio differences, and that the androgen action might be having more of an effect on tissue distribution than bone growth.

More recently, Zhengui Zheng and Martin Cohn at the University of Florida reported a study in mice suggesting that digit ratio is a function of the balance between androgen and estrogen signaling within a particular period of prenatal development. Essentially, the researchers demonstrated differential expression between digits in androgen receptors (AR) and estrogen receptors (ER) that ends up having an effect of chondrocyte (cartilage cell) proliferation. They were able to manipulate digit ratio either by antagonizing the receptors or treating the mice with estrogen or DHT (the precursor to testosterone). ER antagonism masculinized ratios in both males and females (smaller 2D:4D) and AR antagonism feminized the ratios in males. Also, males exposed to estradiol showed feminized ratios and females exposed to DHT showed masculinized ratios. The data also showed that the ratio changes could be accounted for principally by changes in the fourth digit. Perhaps there is some weight to this theory, and perhaps skeletal development is involved…
But why is this interesting? While sex hormones are evolutionarily vital, how significant could the relative lengths of our fingers possibly be? Part of what makes these kinds of studies so cool is that the changes in AR and ER signaling taking place in digit development are occur concurrently with changes in AR and ER signaling that tend to masculinize and feminize the brain. There have been studies that have found evidence for correlation between various behaviors, fertility, sexual orientation etc. and digit ratio. Indeed, there is a lot more biologically based research to be done, but this finger phenomenon is certainly an interesting one. I’ll be looking into more behavioral studies for next time…